The Assessment of Snowpack Enhancement by Silver Iodide Cloud-Seeding using the Physics and Chemistry of the Snowfall
AbstractA new experimental technique is being developed for assessing whether observed changes in the numbers of ice crystals and the quantities of water arriving at the ground at a specific site and in a given time period are related to the seeding of clouds passing over that site. The method, based upon a "source-receptor" concept, involves simultaneous releases from the same "source" locations of ice-nucleating silver iodide complexes and non ice-nucleating indium sesquioxide aerosols into orographic winter storm clouds in the Sierra Nevada. These two aerosols have similar particle size distributions. The concentrations of silver and indiumin the snow samples collected during the seeding periods at the "receptor" sites are measured, together with the numbers of ice crystals per unit water mass in those snow samples. These measurements are used to determine if any observed changes in the ice crystal deposition are attributable to the seeding process. They are also used for estimating whether there have been any associated changes in the quantities of water which have been precipitated to the ground. Preliminary results from the first field trials of the technique are given. The mean minimum enhancement (Emin) in the number of ice crystals deposited during one 8 h seeded storm period at one sampling site in the 1992-93 winter season was 16%. The results suggest that there may have been an associated minimum precipitation enhancement (Pmin) around 3% of the natural precipitation. During a second 5 h seeded storm period in the same winter season, the mean minimum enhancement (Emin) in the number of ice crystals deposited at that same sampling site was 91%. Results suggest that there of may have been an associated minimum precipitation enhancement (Pmin) for this 5 h period around 7% of the natural precipitation. The data from these two seeded storms also showed that the targeting of the aerosols to the sampling site was good; approximately 85% of the snow samples collected contained silver above the background concentration. The proportion of those same samples which contained indium above its background was much lower (average 35%), indicating that more AgI was being incorporated into the precipitation than In2O3. In one of the case studies, it was found that the mean ice crystal mass was higher in the snow samples containing silver above background than in the snow samples where the silver concentration was not above the background 2.0 x 10^-12 g ml^-1. This is consistent with a process involving the production of new ice embryos by the seeding agent followed by aggregation of the ice crystals into larger ice particles. No such differences were found in the second seeded snow storm study. These investigations are continuing in the central Sierra Nevada region of the U.S.A.
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